This invention relates to an improved continuous process for preparing discrete particles from a solution polymer, whereby the size of the particles is controlled to be optimized within a given range, thus enabling rigid filtration and subsequent facile drying of the particles.
A wide variety of polymeric products are currently being marketed for use in such applications as reprographic processes and in hair conditioning and treatment formulations such as that described in U.S. Pat. No. 3,927,199. For ease of handling, in particular, such products are often sold in the form of discrete, spherical particles commonly known in the art as beads or pearls.
The typical manufacture of discrete polymer pearls from polymeric systems involves several steps which are carried out in batch operations. In the first step, selected monomers are solution polymerized in an organic solvent or co-solvent system in the presence of a free-radical catalyst. The solution polymers which result are one-phase systems of high viscosity. In the second step, cold water and a surfactant or protective colloid are added to the non-agitated, hot polymerization system, whereupon agitation is commenced at a prefixed speed. This mixing transforms the solution polymer into a non-viscous suspension of discrete pearls in a continuous aqueous phase. Recovery of the pearls themselves can be accomplished by removing the organic solvent(s) from the suspension and subjecting the remaining pearls to filtration, washing and drying.
The size of the pearls thus obtained can be very important with respect to their subsequent use, particularly in the manufacture of hair treatment products. There is a desirable particle size for any given system depending on the properties required in the final product. For example, it is known that less odor is retained in the final product when the particles, after being washed and subjected to an extended soak and heat cycle, are of a relatively small size. However, a particle size which is too small can cause equipment plugging, especially during filtration and drying, and may retain higher water content in the wet pearl cake. In addition, small particle size may render the emulsion non-filterable. As a further disadvantage, small pearls must be spray-dried as a slurry, a rather costly technique as compared with fluidized drying of the cake, rotary drying and the like, due to the added energy expended on reslurrying and ultimately removing the solvent, usually water, from the system.
The size of the pearls can be controlled by varying such parameters as, for example, the amount of surfactant or protective colloid, agitator speed, agitator type, residence time, and the temperature of the polymeric reaction mixture before water is added thereto. In practice, the parameter most frequently varied in the prior art is the amount of surfactant or colloid, which, when reduced at a given agitator speed and temperature, results in the production of a larger particle size. While the obtaining of such larger particles would theoretically eliminate the problems encountered on filtration and drying as discussed above, these large particles tend to coalesce and coagulate into a large mass during solvent removal, due to the absence of sufficient protective colloid or surfactant present. The purpose of the surfactant or protective colloid is to prevent the coagulation by generating suspended particles which are not too large and which remain discrete under the given conditions of mixing and heat.
In lieu of the surfactant and/or protective colloid, a neutralizing agent, which is generally a common base, is often employed to impart certain properties to the pearls. The neutralizing agent, which serves to neutralize any free carboxyl groups remaining in the solution polymer, is particularly desirable in the preparation of hair treatment products where the polymers should be transparent (non-hazy) when ultimately incorporated into the hair care formulations. However, use of relatively large amounts of neutralizing agent results in a non-filterable emulsion, and use of smaller amounts, as with the surfactant and protective colloid, leads to system instability.
Accordingly, it is an object of the present invention to provide an improved, continuous process for increasing discrete particle size of solution polymers to a controlled level without accompanying filtration or drying problems and without adversely affecting the stability of the system.